Mattress innersprings, made of matrices or arrays of a plurality of wire form springs or coils, have long been used as the reflexive core of a mattress, which is covered with padding and upholstery to complete a mattress. Innersprings made of formed steel wire are mass produced by machinery which forms the coils from steel wire stock and interconnects or laces the coils together in the matrix array. With such machinery, design attributes of innersprings can be selected and modified, including the gauge of the wire, the coil design or combinations of designs, coil orientation relative to adjacent coils in the matrix array, and the manner of interconnection or lacing of the coils.
There are general design considerations of manufacture and comfort which underlie the design of any mattress. For example, considerable effort has been devoted in the industry to the development of coils with end or terminal convolutions which facilitate the interengagement of the spring coils. For example, end convolutions have been developed having offset portions formed thereon which include a straight portion, such as those disclosed in U.S. Pat. Nos. 4,726,572 and 7,404,223. Offset portions enable the spring ends to be secured along a substantial length of the straight portion which will engage with more helical spirals of a lacing wire, and thereby provide more stability for the individual coils. Improved interengagement of the coils of an innerspring without interference and lateral stability is always being sought. Also, ease of manufacture and minimization of costs are always a concern.
An example of a coil which is depicted as having terminal ends which terminate on opposite sides of the coil body is shown in U.S. Pat. No. 7,386,897. As described therein, the coil is used in an innerspring which is constructed with borderwires which encircle the top and bottom support surfaces of the innerspring. As described, the borderwires are necessary for the assembly of an innerspring with this type of coil. The disclosed coils are made of high tensile strength wire to minimize the number of convolutions required to maintain performance characteristics. The high tensile strength wire may minimize the amount of material used but high increases material and handling costs and it also introduces a greater amount of wear on the wire forming equipment.